1. Field of the Invention
The present invention relates to a stereotactics apparatus for designating the position of a portion of interest for a surgical operation in stereotaxy on the basis of a tomographic image of a predetermined portion of a patient which is obtained by a tomographic imaging apparatus.
2. Description of the Related Art
Stereotaxy is a surgical operation for removing, destroying or discharging a portion affected by an ailment such as a cerebral tumor in nuerosurgical clinics. A position of an ailment (i.e., a surgical target) within a brain is calculated in the form of three-dimensional coordinates on the basis of a tomographic image obtained by tomography. For example, a probe is inserted into the ailment to destroy the portion affected by the ailment.
FIGS. 1 and 2 show one example of conventional stereotactics apparatuses. As shown in FIGS. 1 and 2, a stereotactics apparatus includes frame 1 fixed on a head of a patient and two gauge receptacles 2 fixed on the frame. Marker members 3 (3-1, 3-2 and 3-3) are arranged in each gauge receptacle 2 and are displayed as marker images together with a tomographic image of the head. An operational tool consisting of probe 4 and arc 5 for positioning probe 4 is mounted on frame 1.
This apparatus has frame coordinate system (x,y,z) defined on frame 1. A surgeon inserts probe 4 into surgical target T on the basis of coordinates (x.sub.0,y.sub.0,z.sub.0) of surgical target T in the frame coordinate system. As an example, origin I.sub.O of the frame coordinate system is defined as an intersection between a line connecting marker members 3-1 of both receptacles 2 and a line connecting marker members 3-2 of both receptacles 2. The x-, y-, and z-axes are defined a shown in FIGS. 1 and 2.
A tomographic image (slice image perpendicular to the z-axis) of the head is obtained by the tomographic imaging apparatus and is displayed as an image, as shown in FIG. 3. This image has an image coordinate system (X,Y,Z). Coordinates (X.sub.0,Y.sub.0,Z.sub.0) of surgical target T in this image coordinate system can be obtained from the image. However, the image coordinate system does not match the frame coordinate system. Therefore, the correspondence between the image coordinate system and the frame coordinate system must be established. More specifically, the frame coordinate system must be defined on the image. For this purpose, marker members 3 on gauge receptacles 2 are utilized.
Marker members 3 are tomographically imaged together with the head and are displayed as marker images 3 together with a tomographic image of the head. The positional relationship between the tomographic image of the head and the marker images is the same as that between the head and the marker images. For this reason, the frame coordinate system (x,y) is reconstructed (reconstruction along the z-axis will be described later in an embodiment) on the image on the basis of the marker images 3 in the same procedures as in setting of the frame coordinate system. Therefore, coordinates (x.sub.0,y.sub.0) of surgical target T are calculated by measurement with a scale on the basis of the reconstructed frame coordinate system. The surgeon can insert probe 4 into surgical target T on the basis of the calculated coordinates.
A magnetic resonance imaging (MRI) apparatus is used as a tomographic imaging apparatus. A point having zero intensities of X-, Y-, and Z-axis gradient fields is defined as the center of the magnetic field. As shown in FIG. 3, (gradient field coordinate system)=(image coordinate system)=(X,Y,Z) is established.
Nonuniform distributions of intensities of static and gradient fields occur at portions away from the center of the magnetic field. The intensity of the static field must be uniform, but is actually nonuniform. The intensity of each gradient field must have linear characteristics i.e., must be in proportion to the position of the corresponding gradient field axis, but is distorted. For these reasons, the intensity of the magnetic field (i.e., static field intensity+each gradient field intensity) does not have linear characteristics i.e., is not in proportion to the position of each gradient field axis, and is distorted. As a result, a reconstructed image is distorted and is often deviated from a position at which the image is theoretically displayed.
The degree of nonuniformity of the magnetic intensity distribution is increased at a position away from the center of the magnetic field. In a conventional stereotactics apparatus, gauge 2 is flat, as shown in FIG. 1. That is, marker members 3 are placed on a flat imaginary plane. For this reason, as shown in FIG. 3, the distances between the center of the magnetic field and marker images 3-1, 3-2 and 3-3 are different from each other. A marker image relatively separated from the center of the magnetic field is distorted greater than that located near the center of the magnetic field and is displayed with a larger positional error. That is, a positional error of the marker image occurs.
The degree of nonuniformity is proportional to the power of 4 to 5 of the radius from the center of the magnetic field. Distortions of two marker images 3-1 and 3-3 on the right side of FIG. 3 are compared. If a distance between marker image 3-3 and the center of the magnetic field is given as r, a distance between marker image 3-1 and the center of the magnetic field is given as .sqroot.2r (an angle defined by marker image 3-3, the center of the magnetic field, and marker image 3-1 is given as 45.degree.). For this reason, marker image 3-1 is distorted by (.sqroot.).sup.n times with respect to marker image 3-3 (n=4 to 5). As a result, the positional error of marker image 3-1 is figured to be (.sqroot.2).sup.n times that of marker image 3-3. For example, marker image 3-1 is deviated, as indicated by a dotted line in FIG. 3.
The marker image positional error does not allow accurate reconstruction of the frame coordinate system. Coordinates of the surgical target in the frame coordinate system cannot often be accurately calculated.
When an X-ray CT is used as a tomographic imaging apparatus, the following problem occurs. A image is displayed on a circular screen in the X-ray CT apparatus due to its structural limitations. In addition, the tomographic image is an enlarged or reduced image.
In order to precisely display the tomographic image, the tomographic image is preferably displayed as an enlarged image as large as possible. In order to reconstruct the frame coordinate system, marker images must be displayed on the screen. For this reason, a magnification is preset such that marker images 3 are displayed near the peripheral edge of the screen, as shown in FIG. 11.
However, in the conventional stereotactics apparatus, marker members are located on the flat imaginary plane. For this reason, a space for marker images 3 must be sufficiently assured between the tomographic image of the head and the peripheral edge of the screen. The magnification of the image must be inevitably small. The tomographic image of the head and the marker images are displayed on the screen in a relatively small size. Precision of the tomographic image of the head is insufficient. In addition, the marker images are displayed in a relatively small size, and the marker images on the screen cannot be accurately read. As a result, the frame coordinate system cannot be accurately obtained.